1. Field of the Invention
The present invention relates to ventilation systems, and more particularly to such systems as are used in industrial settings to contain and exhaust harmful or unwanted gaseous bi-products generated during various types of manufacturing processes. Although the present invention may be used in a wide variety of industrial settings, it is particularly suited for use in the chemical processing of metals.
2. Description of the Prior Art
Many industrial processes generate fumes and gases that are environmentally harmful--both to the surrounding physical plant and to the operating personnel. This is particularly true in the chemical processing of metals. Large vats of especially noxious solutions are used in a variety of processes ranging from the simple metal cleaning and pickling operations to the sophisticated chem-milling, anodizing, and metal plating treatments. These processes normally require a number of separate treating tanks, with the metal workpiece being moved from one tank to the next as the reaction proceeds. To permit easy access to the processing solution for the insertion and removal of the workpiece, all of these tanks have traditionally remained uncovered, with fumes being generated over the entire surface of the solution. In addition, many of the reactant tanks are heated to speed the chemical reactions, thereby generating even larger quantities of fumes upon the insertion of the workpiece into the tank. Thereafter, removal of the workpiece provides additional quantities of fumes as the heated liquid rapidly evaporates from the now-treated (and hot) workpiece. In addition to these peak times of fume generation, there is always the steady-state problem of fumes leaving the open tanks during the 90% of the time that the heated tank is not being used as part of a chemical processing step.
If left unrestrained and/or uncaptured, the saturated, heated fumes are a potentially deadly health hazard to plant personnel, with almost certain long-term exposure risks. Further, these fumes will eventually destroy all of the structural members in the manufacturing facility with which they come into contact. These solutions are, in fact, so corrosive that structural concrete rapidly ages to powder.
The health and labor codes enacted early this century encouraged industry to capture and control these toxic fumes. Since ready access to the tank solution is required during operation, the conventional systems made use of high volume, negative pressure collection hoods located adjacent to the tank. In most cases, these collector hoods were placed opposite one another on the top edge of the chemical tanks.
Developed from the fine-particulate collection methods, sufficient air was to be pulled through the ventilation hoods that, in theory, would capture all fumes escaping from the tank surface. This system, inefficient at best, was impractical for tanks having widths of greater than four feet. For the wider tanks, one of the pair of suction hoods was converted into a forced-air ventilator, with air blown from the hood, across the liquid surface, and into the corresponding exhaust hood. These latter systems, referred to as push-air systems, had the same air circulation entrainment problems of the conventional system, only exacerbated by the positive or forced air flow across the tank surface. Thermals created by the hot liquid tended to deflect the pushed air stream in an upward manner, frequently to a sufficient extent that a significant portion of the pushed air "escaped" over the exhaust hood and out into the surrounding environment. A second problem occurred each time that a workpiece was lowered or raised from the liquid surface. The workpiece acted as an air baffle, causing the pushed air to be randomly deflected--thereby again missing the exhaust hood and being discharged, saturated with fumes, into the surrounding air.
Aside from the practical problem that these systems are not effective "collectors" of fumes, their greatest disadvantages relates to the extremely high volume of air flow necessary to achieve even the most minimal standard levels of fume capture levels. Extremely large amounts of power are required to physically move the enormous quantities of air in circulation through the system. Further, like in any closed circulation system, the removed air also represents a large amount of lost thermal energy to the system, which must be replaced if the processing is to efficiently continue. The replacement air must be either heated or cooled to the appropriate temperature, and heat energy lost from the processing solutions must also be replaced.
In an effort to reduce this large energy demand, various structures have been suggested. As shown in the published United Kingdom Patent Application, No. GB 2,077,419A (published Dec. 16, 1981), a hood or cover plate is provided that lowers over and partly covers the tank surface during an electroplating operation. However, as previously mentioned, a tank is typically in "operation" less than 10% of the time. The '419 United Kingdom application does not address this problem.
A totally enclosed tank would eliminate all emission problems, however the tank must also be enclosed in a manner that permits ready access to the treating solution by the workpiece. The Madwed patent (U.S. Pat. No. 3,106,927) discloses the use of a vapor-condensing chamber 10 enclosed on all sides except for an open bottom. The chamber sits over the treatment tank and accepts a workpiece through access and exit doors formed in side walls of the chamber 10. Air curtains are also provided to reduce fume emissions when the doors are open. This Madwed device functions in many ways as an "air lock", and its semi-permanent mounting greatly reduces the versatility of the process line, since it is designed to accept workpieces from a certain previous location, in Madwed, the workpieces are conveyed to the air lock from a specific previous location on a straight-line conveyor system.
The majority of chemical process installations make use of craneway and/or monorail hoist mechanisms to convey the workloads to and from the treatment tanks. These hoists provide great freedom with respect to providing access to treatment tanks in a random sequential manner (depending upon the process treatment required) regardless of the immediate proximity of the selected processing tanks to one another. The fixed-line conveyance system required by the Madwed device does not provide such freedom. The Barton patent (U.S. Pat. No. 3,444,802) replaces the doors of Madwed with intense, downwardly directed air streams, and mounts the unit on a hoist. The workpiece is raised and lowered while remaining within an "enclosure" formed by side wall plates 37 and the two downwardly directed air curtains. The Vauriac patent (U.S. Pat. No. 3,567,614) provides a similar device, for a slightly different purpose. To protect the workpiece transfer machinery from the chemical fumes, Vauriac teaches the use of an enclosed, part-holding hoist that is provided with positive internal air pressure to prevent the fumes from entering into the enclosed apparatus. Collection of the emanating fumes is left to conventional exhaust systems.
The great mobility provided by hoists has created difficulties when attempting to make modifications in the conventional exhaust systems. The adjustable hoods of the type shown by Rosenak (U.S. Pat. No. 3,205,810) are not practical where a craneway is operating. The Zalkind patent (U.S. Pat. No. 2,939,378) attempts to solve this mobility problem by permitting the ducts to move up and out of the way when a crane must travel through. Connecting the exhaust ventilation system to the hoist ensures that the ventilation system will be where needed, which is adjacent to the workpiece. However, this solution requires a non-conventional type of connection linking the hoist duct to the central exhaust ventilating system.
Although not disclosed in great detail, Vauriac does teach one possible mechanism for providing such a flexible connection, ensuring adequate positive air pressure within the Vauriac enclosed hoist mechanism. The Ludscheidt patent (U.S. Pat. No. 4,389,923) utilizes an elongate stationary duct connected to a hose by displaceable sealing elements. The sealing elements are linked together to sequentially move in an up and down manner and thereby permit passage of the hose while maintaining the seal. A less complex mechanism is proposed by the Naevestad patent (U.S. Pat. No. 4,087,333) wherein a quench car used in coke production is provided with a traveling hood. The top of the hood narrows into an elongate neck, which in turn projects into a slotted exhaust duct. Parallel flexible sealing strips seal the duct around the elongate neck, permitting the neck to laterally move along the slotted duct.
None of the foregoing devices have achieved an adequate solution to the problem of controlling and capturing emissions generated during the chemical processing of metals, or other multi-step chemical processes where mobility of the workpiece is required. Previous attempts have not been able to resolve the conflict between providing a sealing structure that physically contains the generated fumes in a more "positive" manner than by an air curtain, yet permitting the workpiece to be randomly moved to any number of work stations, maintaining the seal integrity at each station.